1. Field of the Invention
The present invention relates generally to methods and apparatuses for forming downhole pressure plugs in a wellbore. More particularly, the present invention relates to methods of forming downhole plugs to seal the wellbore and to transfer stress from a wellbore tool to the wellbore itself. Additionally, the invention is directed to the use of particulate matter plugs to either transfer loads or to seal during completion operations.
2. Description of the Prior Art
It is conventional in the oil and gas industry to seal wellbores using packers, bridge plugs, and the like. Typically, a wellbore tool, such as a packer or bridge plug, is run into the wellbore to a desired location therein. The packer or bridge plug is inflated or otherwise actuated into sealing engagement with the wellbore. Such a seal may be effected to separate regions in the wellbore, to contain fluid pressure either above or below the wellbore tool for fracturing or other well treatment operations, or other conventional reasons.
Conventional wellbore tools have a force threshold beyond which the wellbore tool will fail mechanically, or will lose gripping and sealing engagement with the wellbore, which tends to cause undesirable movement of the wellbore tool within the wellbore. The force threshold typically is defined in terms of a maximum or limiting differential pressure across the wellbore tool that the wellbore tool can withstand without failure or movement in the wellbore.
If the force threshold is exceeded, mechanical failure of the wellbore tool or undesirable movement of the wellbore tool may result. Mechanical failure may result in at least partial inoperability of the wellbore tool. If the wellbore tool is rendered inoperable, the wellbore may be undesirably obstructed, requiring expensive fishing remedial operations. Mechanical failure at least will require expensive and time-consuming repair or replacement of the wellbore tool.
Even if the wellbore tool does not fail and is not otherwise damaged, the wellbore tool may be moved or displaced within the wellbore if the force threshold is exceeded. Such movement or displacement is undesirable because the positioning of the wellbore tool within the wellbore frequently is of great importance;. Also, movement or displacement of the wellbore tool could damage other wellbore tools or the producing formation itself, thereby necessitating fishing, workover, or other remedial wellbore operations.
In secondary recovery operations, such as formation fracturing, reliable and dependable packers and bridge plugs frequently are necessary. Many secondary recovery operations require sealing off or packing a selected formation interval, and introducing extremely high pressure fluids into the selected interval. High-pressure fluids exert extreme axial forces on the packers or bridge plugs used to seal off the interval. Thus, the possibility of exceeding the force threshold of such wellbore tools is very great in formation fracturing, and requires the use of expensive, reinforced, high-pressure rated wellbore tools. High-pressure wellbore tools typically have relatively large cross-sectional diameters, precluding their use in through-tubing operations or operations in otherwise reduced-diameter or obstructed wellbores.
An alternative to high-pressure rated wellbore tools is to plug or seal the wellbore with cement. Cement plugs have a number of drawbacks. Expensive and specialized cementing equipment usually is required to pump cement into the wellbore to form a cement plug. Also, a significant time period must elapse to permit a cement plug to harden or set into a sealing or load-bearing cement plug. Another drawback of cement plugs is that they are relatively permanent, and require expensive and time-consuming milling operations to remove them from the wellbore.
During wellbore completion operations, a variety of wellbore tools are utilized to either transfer loads within the wellbore or to seal flow paths within the wellbore. For example, cement is utilized to secure sections of casing string in a fixed position relative to the borehole. Alternatively, or in supplementation to casing cement, external casing packers are utilized to fix a section of casing in position relative to the borehole. Liner hangers are utilized to seal and couple sections of casing string to one another. Typically, a casing section of radially-reduced dimension is suspended within a larger diameter casing string which is directly above. Generally, liner hangers include a gripping mechanism which allows the weight of the lower string to be transferred laterally to the upper string. Additionally, the liner hangers typically include metal-to-metal or elastomeric sealing elements or a combination of metal-to-metal and elastomeric sealing elements which seal the potential fluid flow path at the junction of the sections of casing strings.
A completion operation typically requires the placement of a tubing string in a concentric position relative to the casing string. Commonly, the tubing string is centralized and fixed in position relative to the casing string by one or more packer elements. Typically, the packers serve the dual purposes of transferring loads laterally and providing a seal in the annular region between the tubing string and the casing string. Also during completion operations, one or more sections of the casing string may be temporarily or permanently plugged to limit or prevent the flow of fluids between particular regions; of the central bore of the tubing string.
In short, a large number of wellbore tools are utilized during completion operations to either transfer load within the wellbore or to provide a seal at a potential fluid flow path. These wellbore tools are generally rather expensive components. Additionally, they are difficult to replace and repair and frequently require the removal or all or a portion of the wellbore tubulars from the wellbore in order to allow workmen to replace a component. When, for example, a tubing string is pulled from a wellbore, the well is typically "killed"; that is, chemical additives are introduced into the well to prevent or limit the flow of hydrocarbons from the wellbore. Oil and gas well operators are generally reluctant to "kill" a well, since there is no guarantee that the well will later resume production at the levels of production prior to the "killing" and work over operations.